Tuning amplifier

ABSTRACT

A tuning amplifier 1 is provided with an oscillation circuit 10 incorporating an amplifier circuit 11 and a feedback circuit 12, an input circuit 14 which inputs signals to the oscillation circuit 10, and an automatic gain control (AGC) circuit 16 which controls the output amplitude of the oscillation circuit 10. When signals are inputted to the oscillation circuit 10 through the input circuit 14, such tuning that only signals having frequencies near the oscillation frequency of the oscillation circuit 10 are allowed to pass through is possible.

TECHNICAL FIELD

The present invention relates to a tuning amplifier permitting only apass of predetermined frequency components.

BACKGROUND ART

For use as a tuning circuit, various configurations using activeelements and reactance elements have hitherto been proposed and put topractical use.

A conventional tuning circuit utilizing an LC resonance for example hasa gain depending on an LC circuit, which gain may vary upon theregulation of a tuning frequency. Furthermore, typical tuning circuitsextract predetermined frequency components from input signals, so thatthere may usually occur an attenuation of a signal amplitude with thegain resulting in 1 or less at the tuning frequency. For this reason, ifamplification is required, a separate amplifier circuit must beconnected thereto for amplifying the signal amplitude.

DISCLOSURE OF THE INVENTION

The present invention was conceived in order to solve such a problem. Itis therefore the object of the present invention to provide a tuningamplifier ensuring a stabilized output amplitude and capable ofamplifying a signal amplitude simultaneously with tuning.

A tuning amplifier in accordance with the present invention comprises anoscillation circuit for performing an oscillating action at apredetermined frequency, the oscillation circuit allowing its output tobe fed back to its input side to form a closed loop; a gain controlcircuit for providing a control of an output amplitude of theoscillation circuit; and an input circuit for feeding signals to a partof the closed loop of the oscillation circuit. In case the oscillationcircuit in its oscillation mode receives a signal having a frequency inthe vicinity of the oscillation frequency, a phenomenon has beenconfirmed that the oscillation output is drawn into the frequency of theinput signal, allowing a predetermined tuning action. Due to aregulation of the output amplitude effected by a gain control circuit inparticular, there occurs no variation in gain even when the tuningfrequency has been altered by varying the oscillation frequency of theoscillation circuit. A regulation of the response speed of the gaincontrol circuit enables the oscillating action to be performed inresponse to various input AC signals such as an AM modulated signal andan FM modulated signal. It has also been confirmed that the aboveoscillating action is actually achieved when the output amplitude of theoscillation circuit is set to a value fairly smaller than that of thepower source voltage, with the amplitude of the input AC signal beingset to a value fairly smaller than this oscillation amplitude. If theinput AC signal amplitude is taken as a criterion, a gain of severaltens of decibels is achieved, allowing the amplification of the signalamplitude simultaneously with the oscillating action.

The tuning amplifier of the present invention employs a PLLconfiguration including the oscillation circuit in the form of a voltagecontrolled oscillation circuit, enabling the stability of the tuningfrequency to be easily achieved. In particular, the above oscillationcircuit performs a predetermined oscillating action in case of no inputsignals so that a PLL control can be provided irrespective of thepresence of the input signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an embodiment of a tuningamplifier;

FIG. 2 is a diagram showing a configuration to PLL control the tuningamplifier in FIG. 1;

FIG. 3 is a circuit diagram showing a first configuration example of thetuning amplifier using the phase-shift type oscillation circuit;

FIG. 4 is a diagram showing the result of measurement of a tuningcharacteristic of the tuning amplifier in FIG. 3;

FIG. 5 is a diagram showing a tuning characteristic obtained when theinput circuit has been connected to connection point A of FIG. 3;

FIG. 6 is a diagram showing a tuning characteristic obtained when theinput circuit has been connected to the connection point B of FIG. 3;

FIG. 7 is a diagram showing a manner of variation in the outputamplitude in case of input to the input circuit of a signal having thesame frequency as the self-oscillation frequency of the tuning amplifierbut having a different amplitude;

FIG. 8 is a diagram showing a detailed configuration of the tuningamplifier employing the PLL configuration;

FIG. 9 is a diagram showing an example in which the feedback circuitincludes three cascaded low pass filters;

FIG. 10 is a diagram showing an example of three cascaded high passfilters each consisting of a resistor and an inductor;

FIG. 11 is a diagram showing an example of three cascaded low passfilters each consisting of a resistor and an inductor;

FIG. 12 is a diagram showing a detailed configuration of an amplifiercircuit capable of replacing the amplifier circuit shown in FIG. 3;

FIG. 13 is a circuit diagram showing a first variant of the oscillationcircuit;

FIG. 14 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is a variant ofa phase shifting circuit in FIG. 13;

FIG. 15 is a circuit diagram showing another configuration of the phaseshifting circuit which includes the LR circuit;

FIG. 16 is a circuit diagram showing a third variant of the oscillationcircuit;

FIG. 17 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is a variant ofa phase shifting circuit in FIG. 16;

FIG. 18 is a circuit diagram showing another configuration of the phaseshifting circuit which includes the LR circuit;

FIG. 19 is a circuit diagram showing a configuration of the oscillationcircuit comprising a transistor based follower circuit disposed in frontof two phase shifting circuits;

FIG. 20 is a circuit diagram showing a configuration of the oscillationcircuit comprising a non inverting circuit disposed in front of twophase shifting circuits;

FIG. 21 is a circuit diagram showing a configuration of the oscillationcircuit comprising two cascaded the phase shifting circuits 310C;

FIG. 22 is a circuit diagram showing a configuration of the oscillationcircuit comprising two cascaded the phase shifting circuits 330C;

FIG. 23 is a circuit diagram showing a configuration of the oscillationcircuit cascaded the phase shifting circuit includes the transistor;

FIG. 24 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is a variant ofa phase shifting circuit in FIG. 23;

FIG. 25 is a circuit diagram showing another configuration of the phaseshifting circuit which includes the LR circuit;

FIG. 26 is a circuit diagram showing a configuration of the oscillationcircuit comprising two cascaded phase shifting circuits 410C;

FIG. 27 is a circuit diagram showing a configuration of the oscillationcircuit comprising two cascaded phase shifting circuits 430C;

FIG. 28 is a circuit diagram showing a variant of the oscillationcircuit;

FIG. 29 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is a variant ofa phase shifting circuit in FIG. 28;

FIG. 30 is a circuit diagram showing another configuration of the phaseshifting circuit which includes the LR circuit;

FIG. 31 is a circuit diagram showing a configuration of the oscillationcircuit comprising two cascaded phase shifting circuits 510C;

FIG. 32 is a circuit diagram showing a configuration of the oscillationcircuit comprising two cascaded phase shifting circuits 530C;

FIG. 33 is circuit diagram of a part necessary for the action of thephase shifting circuit, extracted from the configuration of theoperational amplifier.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described in detailwith reference to the accompanying drawings.

The present invention is characterized in that it allows an oscillationcircuit to perform a feeble oscillation while simultaneously feeding asignal with a frequency close to the oscillation frequency into a partof the oscillation circuit to thereby allow a tuning action having apredetermined Q and gain.

FIG. 1 is a diagram showing a configuration of an embodiment of a tuningamplifier. The tuning amplifier 1 shown in the diagram comprises anoscillation circuit 10 including an amplifier circuit 11 and a feedbackcircuit 12, an input circuit 14 for feeding a signal into theoscillation circuit 10, and an automatic gain control (AGC) circuit 16for providing a control of an output amplitude of the oscillationcircuit 10.

The oscillation circuit 10 has a typical configuration of theoscillation circuit. The amplifier circuit 11 and the feedback circuit12 form a closed loop. By imparting a frequency selective characteristicto either of the amplifier circuit 11 and the feedback circuit 12 or tothe entirety thereof while controlling the gain of the amplifier circuit11, an oscillating action is achieved at a predetermined frequencycorresponding to the above frequency selective characteristic.

The oscillation circuit 10 is generally roughly divided into aphase-shift type, resonance type and other type of oscillation circuit.The phase-shift type oscillation circuit is subdivided into CR type, LRtype and combination type oscillation circuit. The resonance typeoscillation circuit includes Colpitts type, Hartley type and othervarious types.

The input circuit 14 serves to feed a signal to the closed loop formedby the amplifier circuit 11 and the feedback circuit 12. For example, byway of an impedance element having one end connected to a part of theclosed loop, a signal is fed from the exterior of the oscillationcircuit 10.

The AGC circuit 16 controls an output amplitude of the oscillationcircuit 10 to be kept at a certain value. For example, the gain of theamplifier circuit 11 in the oscillation circuit 10 is controlleddepending on the magnitude of the output amplitude so that control isprovided to keep the output amplitude at a certain value.

Action of the thus configured tuning amplifier will now be described. Incase of application of the tuning amplifier 1 shown in FIG. 1 to areceiver, a signal input via the input circuit 14 to the oscillationcircuit 10 could be a variously modulated signal, e.g., an AM modulatedor FM modulated signal.

Description is first made of a state in which no signal is provided asinput via the input circuit 14. This state is equivalent to a state inwhich a part of the closed loop formed from the amplifier circuit 11 andthe feedback circuit 12 in the oscillation circuit 10 is separated fromthe impedance element in the input circuit 14. Thus, the AGC circuit 16is merely connected to the oscillation circuit 10, allowing apredetermined oscillating action. For example, the output amplitude ofthe oscillation circuit 10 is controlled to be several tens of percentor less of a power supply voltage.

Therefore, in case of application to an FM receiver for example, thetuning amplifier 1 issues a sine-wave signal having a predeterminedfrequency even when no FM wave is received (i.e., when no carrier waveexists), thus eliminating the necessity for a squelch circuit or amuting circuit. The same applies to the case of application to an AMreceiver. Even when no AM wave is received, a sine-wave signal with apredetermined frequency is issued from the tuning amplifier 1, so thatexecution of an AM detection on this sine-wave signal results merely inacquisition of a predetermined DC component without causing any noises.

When in such a state that the oscillation circuit 10 stably oscillatesat a predetermined frequency, a signal having a frequency close to theoscillation frequency is fed through the input circuit 14, the tuningamplifier 1 performs an oscillating action extracting only thecomponents in the vicinity of the oscillation frequency from the inputsignal. It has also been confirmed that if the above oscillationfrequency is coincident with the frequency of the input signal, even inthe case of input of a signal with a small amplitude (e.g., of the orderof 1/10 of the oscillation output amplitude), there can be obtained atuned output with an amplitude larger than the oscillation outputamplitude. It is thus possible to perform an amplification of the orderof several tens of decibels simultaneously with tuning.

Furthermore, by varying a ratio between the amplitude of the oscillationoutput of the oscillation circuit 10 controlled by the AGC circuit 16and the amplitude of an input signal or by altering a point to feed theinput signal, it is possible to vary the Q, that is, a passband width ofthe tuning amplifier 1. Alternatively, instead of altering the abovesignal amplitude ratio, an element constant of the impedance element inthe input circuit 14 may be varied.

The oscillation circuit 10 employs an oscillation frequency variableconfiguration so that a tuning frequency variable type tuning amplifier1 can easily be configured. At that time, by virtue of the AGC circuit16 connected to the oscillation circuit 10 for keeping the oscillationamplitude at a certain value, a stable gain can be obtained upon thetuning without suffering any inconveniences e.g., that the gain of thetuning amplifier 1 may sharply change when the frequency is varied.

In case the input signal is an AM modulated signal, there is a need tofetch as a tuning output a carrier wave of e.g., a voice with apredetermined frequency having overlaid AM modulation components.Accordingly, a response speed of the AGC circuit must be regulated so asto ensure that the AGC circuit 16 can suppress a variation in amplitudehaving a frequency less than that of the AM modulation components.

FIG. 2 is a diagram showing a PLL (phase lock loop) configurationincluding the tuning amplifier 1 shown in FIG. 1. The oscillationcircuit 10 in the tuning amplifier 1 shown in FIG. 1 is comprised of avoltage-controlled oscillator capable of varying the oscillationfrequency in response to an externally applied control voltage, wherebythe tuning amplifier 1 can easily be PLL controlled. More specifically,a PLL configuration is employed which includes the oscillation circuit10 in the tuning amplifier 1, a phase comparator (PD) 2, a charge pump(CP) 4 and a low pass filter (LPF) 5, thereby easily allowing the tuningoutput frequency of the tuning amplifier 1 to coincide with theoscillation frequency of an oscillator (OSC) 3.

The tuning amplifier 1 performs its oscillating action by making use ofthe oscillating action of the oscillation circuit 10 in this manner,with the result that this oscillation output can be utilized to providea PLL control even in the absence of input signals.

[First Configuration Example of Tuning amplifier]

Description will then be made of a detailed configuration of the tuningamplifier 1 in case of using a phase-shift type oscillation circuit 10.

FIG. 3 is a circuit diagram showing a first configuration example of thetuning amplifier 1 using the phase-shift type oscillation circuit. Theinput circuit 14 within the tuning amplifier 1 shown in the diagramincludes an input resistor 22 having one end connected to an inputterminal 21. The input terminal 21 receives a signal in the form of amodulated carrier wave. The feedback circuit 12 includes three cascadedhigh pass filters, that is, a high pass filter consisting of a capacitor23 and a resistor 24, a high pass filter consisting of a capacitor 25and a resistor 26, a high pass filter consisting of a capacitor 27 and aresistor 28.

The amplifier circuit 11 on the other hand includes a transistor 29,resistors 30 to 33, a capacitor 34 and a variable resistor 35. A baseterminal of the transistor 29 is connected via the capacitor 34 forblocking a direct current to an output terminal of the feedback circuit12 and is connected to one ends of the resistors 30 and 31. Theseresistors 30 and 31 are provided for applying a bias to the transistor29. The resistor 32 is disposed between a collector terminal of thetransistor 29 and a positive power source, with the resistor 33 and thevariable resistor 35 being disposed between an emitter terminal and aground terminal. The resistance value of the variable resistor 35 isvariable by a control voltage output from the AGC circuit 16.Description is made hereinbelow of a case where the variable resistor 35is comprised of a p-channel type FET.

It is to be noted that a capacitor 35A connected in series to thevariable resistor 35 serves to block a direct current, thus making itpossible to change the gain of the amplifier circuit 11 without varyingan operating point of the transistor 29 when the resistance value of thevariable resistor 35 has been varied.

The AGC circuit 16 includes a transistor 36, resistors 37 to 44,capacitors 45 to 48, and a diode 49. A base terminal of the transistor36 is connected via the capacitor 45 and the resistor 37 to an outputterminal of the amplifier circuit 11 and is connected to one ends of theresistors 38 and 39. These resistors 38 and 39 are provided for applyinga bias to the transistor 36. The resistor 40 is interposed between acollector terminal of the transistor 36 and a positive power source,with the resistor 41 being interposed between an emitter terminal and aground terminal.

Action of the tuning amplifier 1 shown in FIG. 3 will hereinafter bedescribed. Let Vi be an input voltage at the feedback circuit 12, Vo anoutput voltage, C a capacitance of the capacitors 23, 25 and 27, and R aresistance value of the resistors 24, 26 and 28. Then, a relationship isgiven as

    Vo=Vi·(ωCR).sup.3 /[(ωCR).sup.3 -5ωCR-j{6(ωCR).sup.2 -1}]                     (1)

When the imaginary part of the expression (1) is null, the phase-shiftamount of the feedback circuit 12 results in 180°. The frequency of theinput signal in this case is given as

    f=1/2π√ (6CR)                                    (2)

The amplifier circuit 11 shown in FIG. 3 is of an emitter groundedconfiguration, so that the direction of a change in the voltage at thebase terminal of the transistor 29 is opposite to the direction of achange in the voltage at the collector terminal. That is, thephase-shift amount of the amplifier circuit 11 is 180°, with the resultthat the total phase-shift amount of the feedback circuit 12 and theamplifier circuit 11 becomes equal to 360° at a predetermined frequency.Furthermore, the output of the amplifier circuit 11 is fed back to theinput side of the feedback circuit 12 so that the tuning amplifier 1 ofFIG. 3 oscillates in a stabilized manner as long as the combined gain ofthe feedback circuit 12 and the amplifier circuit 11 is equal to 1.

Although three high pass filters are cascaded in FIG. 3, the number ofhigh pass filters to be cascaded may naturally be more than three.

Once the feedback circuit 12 receives a signal having a frequency givenby the expression (2), the output voltage Vo at the feedback circuit 12attenuates to about 1/29 as apparent from expression (3).

    Vo=Vi×(1/√ 6).sup.3 /{(1/√ 6).sup.3 -5/√ 6}≈-0.0344 Vi≈-1/29                       (3)

For this reason, if the amplifier circuit 11 has a 29 times voltagegain, the entire gain of the tuning amplifier 1 becomes equal to 1,allowing a stable oscillation even though no signal is fed via the inputcircuit 14. If, when the tuning amplifier 1 oscillates at apredetermined frequency, a signal having a frequency close to theoscillation frequency is fed via the input circuit 14, then the tuningamplifier 1 performs an oscillating action extracting only thecomponents in the vicinity of the oscillation frequency.

On the contrary, the output from the amplifier circuit 11 is fed to thebase terminal of the transistor 36 by way of the DC blocking capacitor45 and the resistor 37 within the AGC circuit 16. Accordingly, only theAC components of the output from the amplifier circuit 11 are amplifiedby the transistor 36. This amplified output is rectified by the diode 49and then passed through a smoothing circuit consisting of the capacitors47 and 48 and the resistor 43, thus obtaining a predetermined controlvoltage. By virtue of such an action of the AGC circuit 16, if theoutput amplitude of the amplifier circuit 11 becomes larger, the controlvoltage is boosted to heighten the resistance value of the variableresistor 35 comprised of a p-channel type FET to lessen the outputamplitude of the amplifier circuit 11, whereas if the output amplitudeof the amplifier circuit 11 becomes smaller, the control voltage isreduced to lower the resistance value of the variable resistor 35 toenlarge the output amplitude of the amplifier circuit 11, therebykeeping the output amplitude of the amplifier circuit 11 atsubstantially a constant level.

Description will then be made of the result of experiments on thecircuit of FIG. 3 which has actually been put together. It is to benoted that the experimental result set forth hereinbelow was obtained byuse of the tuning amplifier 1 allowing the oscillation circuit 10 tooscillate at a predetermined amplitude without providing an amplitudecontrol by the AGC circuit 16.

When a self-oscillation output from the tuning amplifier 1 has anamplitude of about 900 mV in the absence of a signal fed to the inputcircuit 14, the input terminal 21 received an AC signal having anamplitude of 75 mV and the same frequency as the self-oscillationfrequency. As a result of this, the output amplitude of the tuningamplifier 1 was of the order of 3 Vpp. Accordingly, the gain in thiscase resulted in about 32 dB as given by expression (4). It has provedthat a sufficient gain was obtained. Note that the power source voltagewas 9V.

    GAIN=20×log (3/0.075)=20 ×1.602=32.04          (4)

FIG. 4 is a diagram showing the result of measurement of a tuningcharacteristic of the tuning amplifier 1 illustrated in FIG. 3. The axisof abscissas represents frequencies of the input signal and the axis ofordinates represents amplitudes of the oscillation output. As can beseen from FIG. 4, the tuning amplifier 1 has a tuning characteristichaving as a peak the same frequency component as the self-oscillationfrequency.

On the other hand, FIGS. 5 and 6 are diagrams showing tuningcharacteristics obtained when altering a point to connect the inputcircuit 14. More specifically, FIGS. 5 and 6 respectively illustrate thetuning characteristics obtained when the input circuit 14 has beenconnected to connection points A and B of FIG. 3. As is apparent fromthe FIGS. 5 and 6, a variation in the value of Q is caused by a changeof the point to connect the input circuit 14, in other words, by achange of the point to feed a signal. Accordingly as the point to feedthe input signal comes away from the amplifier circuit 11, the value ofQ becomes larger.

A switch or the like may therefore be provided so as to allow theconnection point of the input circuit 14 to be arbitrarily changeable.Thus, if an improvement in selectivity of the received frequency isdesired in case of using the tuning amplifier 1 as a receiver, forexample, if adjacent frequencies include frequency signals from otherstations, the value of Q may be increased so that the selectivity caneasily be improved. On the contrary, if the adjacent frequencies includeno frequency signals from other stations, the value of Q may bediminished so that the reproducibility can easily be improved.

It is to be appreciated that the value of Q may be varied by changingthe resistance value of the input resistor 22 instead of changing theinput signal feeding point. Therefore, the input resistor 22 may bereplaced by a variable resistor capable of change of the resistancevalue so as to ensure an easier regulation of the value of Q than thecase of change of the input signal feeding point. Alternatively, thevalue of Q could be regulated by varying a ratio between the inputsignal amplitude and the output amplitude of the amplifier circuit 11controlled by the AGC circuit 16.

FIG. 7 is a diagram showing a manner of variation in the outputamplitude in case of input to the input circuit 14 of a signal havingthe same frequency as the self-oscillation frequency of the tuningamplifier 1 but having a different amplitude. As shown in the diagram, avariation in the amplitude of the signal input to the input circuit 14results in a substantially linear variation of the output amplitude(unit: dB) of the tuning amplifier 1. Thus, according to the tuningamplifier 1 of this embodiment, it is possible to faithfully fetch avariation in amplitude arising from AM modulation, etc., contained inthe carrier wave components.

[Second Configuration Example of Tuning amplifier]

The tuning amplifier 1 shown in FIG. 3 is not configured on theassumption that it employs a PLL configuration, although it may employthe PLL configuration as shown in FIG. 2. FIG. 8 is a diagram showing adetailed configuration of the tuning amplifier employing the PLLconfiguration. The tuning amplifier generally designated at 1A in thediagram differs from the tuning amplifier 1 shown in FIG. 3 only in thatthe resistors 24, 26 and 28 within the feedback circuit 12 are replacedby variable resistors 61, 62 and 63, respectively. Resistance values ofthese variable resistors 61 to 63 are varied in response to the outputfrom the low pass filter 5 shown in FIG. 2, whereby a control isprovided such that the frequency of the oscillation output of the tuningamplifier 1A is coincident with the oscillation frequency of theoscillator 3 shown in FIG. 2.

[Third Configuration Example of Tuning amplifier]

Although the feedback circuit 12 shown in FIG. 3 includes three cascadedhigh pass filters, the high pass filters may be replaced by low passfilters in cascade connection. FIG. 9 illustrates an example in whichthe feedback circuit 12 includes three cascaded low pass filters eachconsisting of a resistor and a capacitor. In contrast with the high passfilter, the low pass filter has a nature delaying the phase, so that thephase-shift amount of the entire feedback circuit 12 becomes equal to180° at a predetermined frequency. It is therefore possible to obtainthe same frequency selective characteristic as the case of the cascadeconnection of the high pass filters.

On the other hand, FIG. 10 illustrates an example of three cascaded highpass filters each consisting of a resistor and an inductor, and FIG. 11illustrates an example of three cascaded low pass filters eachconsisting of a resistor and an inductor. Although they have differentphase shifting directions from each other, the two examples are the samein that the phase-shift amount of the entire feedback circuit 12 becomesequal to 180° when the input signal frequency is equal to thepredetermined frequency, thus allowing a replacement of the feedbackcircuit 12 shown in FIG. 3 therewith.

It is to be noted that in case of the replacement the number of cascadedhigh pass filters or low pass filters is not limited to three. In casethe entire feedback circuit 12 including the inductors is formed on thesemiconductor substrate, the inductance of each inductor is extremelyreduced, with the result that the oscillation frequency of theoscillation circuit 10, namely, the tuning frequency of the tuningamplifier 1 goes very high. In such a case, it is preferred that theinput circuit 14 of FIG. 3 be formed from an inductor in place of theinput resistor 22.

In case no amplitude control is provided by the AGC circuit 16, beatsmay occur if there is a slight different between the self-oscillationfrequency and the input signal frequency. The occurrence of such beatscan be prevented by the provision of amplitude control by the AGCcircuit 16.

[Fourth Configuration Example of Tuning amplifier]

FIG. 12 is a diagram showing a detailed configuration of an amplifiercircuit 11A capable of replacing the amplifier circuit 11 shown in FIG.3. The amplifier circuit 11A shown in the diagram includes a CMOSinverter 54 and resistors 55 and 56. The CMOS inverter 54 acts as ananalog amplifier by connecting its input and output via the resistor 56.The gain at that time is determined depending on a ratio of theresistance between the resistors 55 and 56. Let R55 and R56 beresistance values of the resistors 55 and 56, respectively, then thegain of the amplifier circuit 11A is expressed as R56/R55. Thus, forexample, by connecting the variable resistor 35 shown in FIG. 3 inparallel to the resistor 55 or 56 to regulate the resistance value inresponse to the output of the AGC circuit 16, it is possible to suppressa variation in amplitude of the oscillation output in the same manner asthe case of the tuning amplifier 1 shown in FIG. 3. An example of suchconnection of variable resistor 35 is illustrated in phantom in FIG. 12.

In case of using the amplifier circuit 11A shown in FIG. 12 toconstitute the tuning amplifier 1 in this manner, a CMOS process basedfabrication becomes possible, achieving a simplification ofmanufacturing processes as well as a reduction in costs.

[First Variant of Oscillation Circuit]

FIG. 13 is a circuit diagram showing a first variant of the oscillationcircuit. The oscillation circuit generally designated at 10A in thediagram comprises two phase shifting circuits 110C and 130C, at voltagedividing circuit 160 and a feedback resistor 170. The phase shiftingcircuits 110C and 130C serve to allow the phase of an input AC signal toshift by a predetermined amount. The combined phase-shift amount of thetwo phase shifting circuits 110C and 130C are set to 360° at apredetermined frequency. The voltage dividing circuit 160 is disposed onthe output side of the posterior phase shifting circuit 130C andconsists of resistors 162 and 164.

The anterior phase shifting circuit 110C includes a capacitor 114 andresistors 116, 118, 120, 121 and 123. An inverting input terminal of anoperational amplifier 112 receives an AC signal byway of the resistor118, while a non-inverting input terminal of the operational amplifier112 connects with a CR circuit consisting of a capacitor 114 and aresistor 116. An output terminal of the operational amplifier 112 isassociated with a voltage dividing circuit consisting of the resistors121 and 123. The resistor 120 is interposed between a divided voltageoutput terminal of the voltage dividing circuit and the inverting inputterminal of the operational amplifier 112. The resistor 118 and theresistor 120 are set to have the same resistance value.

A transfer function K1 of the anterior phase shifting circuit 110Chaving such a configuration is given as

    K1=-a.sub.1 (1-T.sub.1 s)/(1+T.sub.1 s)                    (5)

where T₁ is a time constant of the CR circuit consisting of the resistor116 and the capacitor 114, s=jω), and a₁ is a gain of the phase shiftingcircuit 110C expressed as a₁ =(1+R21/R23)>1. Note that R21 and R23 areresistance values of the resistors 121 and 123, respectively.

As is apparent from the expression (5), the phase shifting circuit 110Cis an full band pass circuit, so that the output amplitude of its outputvoltage Eo is constant irrespective of the frequency, allowing thephase-shift amount .O slashed.1 to vary from 180° to 360° in response tothe frequency of the input signal. The phase shifting circuit 110C iscapable of providing a gain larger than 1 by regulating the resistancevalues R21 and R23.

On the other hand, the posterior phase shifting circuit 130C shown inFIG. 13 includes an operational amplifier 132, a CR circuit consistingof a capacitor 134 and a resistor 136, a voltage dividing circuitconsisting of resistors 141 and 143, a resistor 138 and a resistor 140.

The circuit elements constituting the phase shifting circuit 130C areconnected in the same manner as the phase shifting circuit 110C exceptthat a way of connection of the capacitor 134 and the resistor 136 isopposite to the case of the phase shifting circuit 110C, with theresistors 138 and 140 being set to have the same resistance values.

A transfer function K2 of the posterior phase shifting circuit 130Chaving such a configuration is given as

    K2=a.sub.2 (1-T.sub.2 s)/(1+T.sub.2 s)                     (6)

where T2 is a time constant of the CR circuit consisting of thecapacitor 134 and the resistor 136, s=jω, and a₂ is a gain of the phaseshifting circuit 130C expressed as a₂ =(1+R41/R43)>1. Note that R41 andR43 are resistance values of the resistors 141 and 143, respectively.

As is apparent from the expression (6), the phase shifting circuit 130Cis an full band pass circuit, so that the output amplitude of its outputvoltage Eo is constant irrespective of the frequency, allowing thephase-shift amount .O slashed.2 to vary from 0° to 180° in response tothe frequency of the input signal. The phase shifting circuit 130C iscapable of providing a gain larger than 1 by regulating the resistancevalues R41 and R43.

In this manner, the phase is shifted by a predetermined amount in boththe phase shifting circuits 110C and 130C, with the result that thecombined phase-shift amount of the two phase shifting circuits 110C and130C becomes equal to 360° at a predetermined frequency. Thus, bysetting to 1 or more the loop gain of the closed loop including the twophase shifting circuits 110C and 130C, the oscillation circuit 10A isallowed to oscillate at this predetermined frequency.

The posterior phase shifting circuit 130C is followed by the voltagedividing circuit 160, with a variable resistor 166 being connected inparallel to the resistor 164 constituting the voltage dividing circuit160. The variable resistor 166 is formed from for example a channelresistor of a p-channel type FET. A gate terminal of this FET isassociated with the output terminal of the AGC circuit 16 shown in FIG.3. Thus, for example, when the output amplitude of the oscillationcircuit 10A shown in FIG. 13 becomes larger, the control voltage outputfrom the AGC circuit 16 shown in FIG. 3 is built up, allowing the gatevoltage at the FET to increase. For this reason, the resistance value ofthe variable resistor 166 increases so that control is provided so as toreduce the output amplitude of the oscillation circuit 10A.

On the contrary, when the output amplitude of the oscillation circuit10A becomes smaller, the control voltage output from the AGC circuit 16lowers and the resistance value of the variable resistor 166 diminishes,with the result that control is provided so as to enlarge the outputamplitude of the oscillation circuit 10A. By virtue of such control, theoutput amplitude of the oscillation circuit 10A is at all times kept ata certain value.

In this manner, the oscillation circuit 10A shown in FIG. 13 uses as afeedback signal a signal attenuated via the voltage dividing circuit 160and fetches as an output from the oscillation circuit 10A a signalprevious to the input to the voltage dividing circuit 160, whereby it ispossible to perform an oscillating action extracting only apredetermined frequency component from among the input signals and toperform a predetermined amplifying action on the thus extracted signal.In addition, due to the fact that the voltage dividing ratio of thevoltage dividing circuit 160 is varied in response to the output fromthe AGC circuit 16 shown in FIG. 3, the oscillation circuit can havesubstantially a constant output amplitude. Furthermore, the oscillationcircuit 10A is formed from a combination of the operational amplifiers,the capacitors and resistors in order that all the constituent elementscan be formed on the substrate.

Although the resistance values of the feedback resistor 170 and of theinput resistor 22 within the input circuit 14 are fixed in FIG. 13, atleast one of them may be replaced by a variable resistor so as to allowan alteration of the ratio of resistance between the feedback resistor170 and the input resistor 22. This makes it possible to regulate Q ofthe tuning amplifier 1.

Furthermore, the resistor 116 and/or the resistor 136 may be replaced bya variable resistor so as to allow an alteration of the time constant ofthe CR circuit within the phase shifting circuit 110C or 130C. Acontinuous alteration of the resistance value of the resistor 116 and/orthe resistor 136 will enable the oscillation frequency to continuouslychange.

It is to be appreciated that upon the cascade connection of the phaseshifting circuits 110C and 130C, either of the voltage dividing circuitsconnected to the output terminals of the operational amplifiers 112 and132 in the respective phase shifting circuits may be excluded oralternatively the voltage dividing ratio may be set to 1.

Although the oscillation circuit 10A shown in FIG. 13 alters a voltagedividing ratio of the voltage dividing circuit 160 in response to thecontrol voltage output from the A(;C circuit 16, there may be altered inresponse to the control voltage from the AGC circuit 16 the voltagedividing ratio of the voltage dividing circuit in the phase shiftingcircuit 110C and/or the voltage dividing ratio of the voltage divingcircuit in the phase shifting circuit 130C. In this case, the voltagedividing circuit 160 posterior to the phase shifting circuit 130C may beexcluded such that the output from the phase shifting circuit 130C isfed directly back to the anterior stage side. Alternatively, the voltagedividing ratio may be set to 1 with the resistance value of the resistor162 in the voltage dividing circuit 160 being set to an extremely smallvalue.

In the case of the oscillation circuit 10A shown in FIG. 13, an inputsignal is fed to the input circuit 14 disposed in front of the anteriorphase shifting circuit 110C, although the point to feed the input signalis not limited to the front side of the anterior phase shifting circuit110C, but instead the input signal may be fed for example to the inputcircuit 14 interposed between the phase shifting circuit 110C and thephase shifting circuit 130C.

[Second Variant of Oscillation Circuit]

Although the oscillation circuit 10A shown in FIG. 13 comprises thephase shifting circuits each including a CR circuit therewithin, theoscillation circuit may comprise a phase shifting circuit including anLR circuit in place of the CR circuit.

FIG. 14 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is replaceablewith the anterior phase shifting circuit 110C of the oscillation circuit10A shown in FIG. 13. The phase shifting circuit designated at 110L inthe diagram includes an LR circuit consisting of a resistor 116 and aninductor 117, in lieu of the CR circuit consisting of the capacitor 114and the resistor 116 within the anterior phase shifting circuit 110Cshown in FIG. 13. The other configurations are substantially the same asthose of the phase shifting circuit 110C. The phase shifting circuit110L has a transfer function and phase-shift amount identical to thoseof the phase shifting circuit 110C.

FIG. 15 is a circuit diagram showing another configuration of the phaseshifting circuit which includes the LR circuit and which is replaceablewith the posterior phase shifting circuit 130C of the oscillationcircuit 10A shown in FIG. 13. The phase shifting circuit designated at130L in the diagram includes an LR circuit consisting of a resistor 136and an inductor 137, in lieu of the CR circuit consisting of thecapacitor 134 and the resistor 136 within the posterior phase shiftingcircuit 130C shown in FIG. 13. The other configurations aresubstantially the same as those of the phase shifting circuit 130C. Thephase shifting circuit 130L has a transfer function and phase-shiftamount identical to those of the phase shifting circuit 130C.

In this manner, the phase shifting circuit 110L shown in FIG. 14 isequivalent to the phase shifting circuit 110C shown in FIG. 13, with aphase shifting circuit 130L shown in FIG. 15 being equivalent to thephase shifting circuit 130C shown in FIG. 13. Therefore, the phaseshifting circuit 110C and/or the phase shifting circuit 130C can bereplaced by the phase shifting circuit 110L and/or the phase shiftingcircuit 130L. In case both the phase shifting circuits 110C and 130C arereplaced by the phase shifting circuits 110L and 130L, the entire tuningamplifier may be integrated so that a higher oscillation frequency caneasily be acquired.

In case either of the two phase shifting circuits 110C and 130C isreplaced by the phase shifting circuit 110L or 130L correspondingthereto, the entire tuning amplifier including the inductor making upthe LR circuit or excluding the inductor may be integrated so that itbecomes possible to prevent a variation in the oscillation frequencyattributable to a change of the temperature, that is, to effect aso-called temperature compensation.

Besides, in case the phase shifting circuit 110C and/or the phaseshifting circuit 130C shown in FIG. 13 are replaced by the phaseshifting circuit 110L of FIG. 14 and/or the phase shifting circuit 130Lof FIG. 15, either of the voltage dividing circuits connected to theoutput terminals of the operational amplifiers 112 and 132 in therespective phase shifting circuits may be eliminated or alternativelymay be set to have a voltage dividing ratio equal to 1.

[Third Variant of Oscillation Circuit]

FIG. 16 is a circuit diagram showing a third variant of the oscillationcircuit. The oscillation circuit designated at 10B in the diagramcomprises an anterior phase shifting circuit 210C including no voltagedividing circuit therewithin. Instead, the resistance value of aresistor 120' is set to be larger than that of a resistor 118' so thatthe phase shifting circuit 210C can have a gain larger than 1. The otherconfigurations are substantially the same as those of the phase shiftingcircuit 110C shown in FIG. 13, with the transfer function and thephase-shift amount thereof being basically the same as those of thephase shifting circuit 110C.

In the same manner, a posterior phase shifting circuit 230C includes novoltage dividing circuit therewithin. Instead, the resistance value of aresistor 140' is set to be larger than that of a resistor 138' so thatthe phase shifting circuit 230C can have a gain larger than 1. The otherconfigurations are substantially the same as those of the phase shiftingcircuit 130C shown in FIG. 13, with the transfer function and thephase-shift amount thereof being basically the same as those of thephase shifting circuit 130C.

The output of the phase shifting circuit 230C is fed back to theanterior stage side by way of the voltage dividing circuit 160, with thevariable resistor 166 connected in parallel to the resistor 164 makingup the voltage dividing circuit 160. This variable resistor 166 isformed for example from a channel resistor of an FET. In response to thecontrol voltage output from the AGC circuit 16 shown in FIG. 13, thegate voltage at the FET is regulated, in response to which the FETchannel resistance is altered.

Resistors 119 and 139 are provided for suppressing a variation in thegain of the phase shifting circuits 210C and 230C, respectively, andpreferably have respective resistance values R of the resistor 119 andresistor 139 given by expression (7) where r and mr denote resistancevalues of the resistors 118' and 120', respectively.

    R=mr/(m-1)                                                 (7)

Although the oscillation circuit 10B shown in FIG. 16 prevents avariation in the amplitude upon a change of the oscillation frequency bythe connection of the resistors 119 and 139 to the phase shiftingcircuits 210C and 230C, respectively, the oscillation circuit may beconfigured without using the resistors 119 and 139 descried above.Alternatively, either the resistor 119 or the resistor 139 may merely beremoved to configure the oscillation circuit.

In the oscillation circuit 10B shown in FIG. 16 the voltage dividingratio of the voltage dividing circuit 160 is altered in response to thecontrol voltage from the AGC circuit 16, although alteration may be madeof a resistance ratio of the resistors 118' and 120' and/or a resistanceratio of the resistors 138' and 140' in response to the control voltagefrom the AGC circuit 16.

In the case of the oscillation circuit 10B shown in FIG. 16, an inputsignal is fed to the input circuit 14 disposed in front side of theanterior phase shifting circuit 210C, although the point to feed theinput signal is not limited to the front side of the anterior phaseshifting circuit 210C, but instead the input signal may be fed forexample to the input circuit 14 interposed between the phase shiftingcircuit 210C and the phase shifting circuit 230C.

[Fourth Variant of Oscillation Circuit]

Although the oscillation circuit 10B shown in FIG. 16 comprised thephase shifting circuits each including the CR circuit, the oscillationcircuit may comprise a phase shifting circuit including an LR circuittherewithin instead of the CR circuit.

FIG. 17 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is replaceablewith the anterior phase shifting circuit 210C of the oscillation circuit10B shown in FIG. 16. The phase shifting circuit designated at 210L inthe diagram includes an LR circuit consisting of a resistor 116 and aninductor 117, in lieu of the CR circuit consisting of the capacitor 114and the resistor 116 within the anterior phase shifting circuit 210Cshown in FIG. 16. The other configurations are substantially the same asthose of the phase shifting circuit 210C. The phase shifting circuit210L has a transfer function and phase-shift amount identical to thoseof the phase shifting circuit 210C.

On the other hand, FIG. 18 is a circuit diagram showing anotherconfiguration of the phase shifting circuit which includes the LRcircuit and which is replaceable with the posterior phase shiftingcircuit 230C of the oscillation circuit 10B shown in FIG. 16. The phaseshifting circuit designated at 230L in the diagram includes an LRcircuit consisting of an inductor 137 and a resistor 136, in lieu of theCR circuit consisting of the resistor 136 and the capacitor 134 withinthe posterior phase shifting circuit 230C shown in FIG. 16. The otherconfigurations are substantially the same as those of the phase shiftingcircuit 230C. The phase shifting circuit 230L has a transfer functionand phase-shift amount identical to those of the phase shifting circuit230C.

In this manner, the phase shifting circuit 210L shown in FIG. 17 isequivalent to the phase shifting circuit 210C shown in FIG. 16, with aphase shifting circuit 230L shown in FIG. 18 being equivalent to thephase shifting circuit 230C shown in FIG. 16. Therefore, the phaseshifting circuit 210C and/or the phase shifting circuit 230C shown inFIG. 16 can be replaced by the phase shifting circuit 210L and/or thephase shifting circuit 230L.

In case both the phase shifting circuits 210C and 230C are replaced bythe phase shifting circuits 210L and 230L, the entire tuning amplifiermay be integrated so that a higher oscillation frequency can easily beacquired. In case either of the two phase shifting circuits 210C and230C is replaced by the phase shifting circuit 210L or 230Lcorresponding thereto, so that it becomes possible to prevent avariation in the oscillation frequency attributable to a change of thetemperature.

[Fifth Variant of Oscillation Circuit]

The first to fourth variants of the oscillation circuit may employ afollower circuit in the form of a transistor connected to a part of theclosed loop formed from two cascaded phase shifting circuits.

FIG. 19 is a circuit diagram showing a configuration of a fifth variantof the oscillation circuit. The oscillation circuit designated at 10C inthe diagram comprises a transistor based follower circuit 50 disposedanterior to the anterior phase shifting circuit 110C of the oscillationcircuit 10A shown in FIG. 13.

The follower circuit 50 includes an FET 52 having a drain connected to apositive power source Vdd and a source connected via a resistor 53 to anegative power source Vss. It is to be appreciated that the followercircuit 50 may be formed from an emitter follower circuit instead of thesource follower circuit as shown in FIG. 19.

Thus, by cascading the transistor based follower circuit in front of theanterior phase shifting circuit 110C, etc., there can be compensated fora loss arising from the input impedance of the anterior phase shiftingcircuit 110C, etc., thereby making it possible to increase theresistance values of the feedback resistor 170 and the input resistor 22as compared with the oscillation circuit 10A, etc., shown in FIG. 13,etc. In case the oscillation circuit 10C and other circuits areintegrated on the semiconductor substrate in particular, a largerelement occupancy area is required to diminish the resistance value ofthe feedback resistor 170, etc. It is therefore desired that theresistance value of the feedback resistor 170, etc., be increased to acertain extent by the provision of the follower circuit.

Although the oscillation circuit 10C shown in FIG. 19 alters a voltagedividing ratio of the voltage dividing circuit 160 in response to thecontrol voltage output from the AGC circuit 16, there may be altered inresponse to the control voltage from the AGC circuit 16 the voltagedividing ratio of the voltage dividing circuit in the phase shiftingcircuit 110C and/or the voltage dividing ratio of the voltage divingcircuit in the phase shifting circuit 130C.

In the case of the oscillation circuit 10C shown in FIG. 19, an inputsignal is fed to the input circuit 14 disposed in front of the anteriorphase shifting circuit 110C, although the point to feed the input signalis not limited to the front side of the anterior phase shifting circuit110C, but instead the input signal may be fed for example to the inputcircuit 14 interposed between the phase shifting circuit 110C and thephase shifting circuit 130C. In this case, it is desired that thefollower circuit 50 shown in FIG. 19 be interposed between the phaseshifting circuit posterior to the input circuit 14 and the input circuit14.

[Sixth Variant of Oscillation Circuit]

Although the oscillation circuit 10A shown in FIG. 13 has a 360°combined phase-shift amount of the two phase shifting circuits 110C and130C, the oscillation circuit may comprise a non-inverting circuitallowing no phase shifting connected to the cascaded phase shiftingcircuits 110C and 130C.

FIG. 20 is a circuit diagram showing a configuration of the oscillationcircuit 10D comprising a non-inverting circuit 150 disposed in front oftwo phase shifting circuits. As shown in the diagram, the phase shiftingcircuits designated at 310C and 330C within the oscillation circuit 10Dhave the same configurations as those of the phase shifting circuits110C and 130C shown in FIG. 13 except that no voltage dividing circuitis connected to the output terminals of the operational amplifiers 112and 132. The transfer function and the phase-shift amount are alsoidentical to those of the phase shifting circuits 110C and 130C. It isto be noted that a₁ =1 in the expression (5) and that a₂ =1 in theexpression (6).

The non-inverting circuit 150 includes an operational amplifier 152having a non-inverting input terminal for receiving an AC signal and aninverting input terminal connected via a resistor 154 to the ground, anda resistor 156 interposed between the inverting input terminal and theoutput terminal of the operational amplifier 152. The non-invertingcircuit 150 has a predetermined gain determined depending on theresistance ratio between the two resistors 154 and 156.

The two phase shifting circuits 310C and 330C have a gain equal to 1 incommon. Therefore, instead of achieving the gain by each phase shiftingcircuit, the oscillation circuit 10D shown in FIG. 20 sets the gain ofthe non-inverting circuit 150 to a value larger than 1 so that theclosed loop including the two phase shifting circuits 310C and 330C canacquire a loop gain equal to or more than 1, thereby allowing apredetermined oscillation.

Although the oscillation circuits 10D shown in FIG. 20 vary the voltagedividing ratio of the voltage dividing circuit 160 in response to thecontrol voltage output from the AGC circuit 16, the gain of thenon-inverting circuit 150 may be varied in response to the controlvoltage from the AGC circuit 16.

[Seventh Variant of Oscillation Circuit]

The above-described oscillation circuits 10A, 10B, 10C and 10D performeda predetermined oscillating action at a frequency allowing the combinedphase-shift amount of the two phase shifting circuits to become equal to360°. However, the oscillation circuit may comprise a combination of twophase shifting circuits performing basically the same action so that apredetermined oscillating action is achieved at a frequency allowing thecombined phase-shift amount of the two phase shifting circuits to becomeequal to 180°.

FIG. 21 is a circuit diagram showing a seventh variant of theoscillation circuit. The phase shifting circuit 310C is provided inplace of the posterior phase shifting circuit 330C shown in FIG. 20,with a phase inverting circuit 180 in lieu of the non-inverting circuit150.

The phase inverting circuit 180 includes an operational amplifier 182having an inverting input terminal receiving an input AC signal by wayof a resistor 184 and having a grounded non-inverting input terminal,and a resistor 186 disposed between the inverting input terminal and theoutput terminal of the operational amplifier 182. When an AC signal isfed via the resistor 184 to the inverting input terminal of theoperational amplifier 182, the output terminal of the operationalamplifier 182 issues an anti phase signal having an inverted phase,which signal in turn is fed to the anterior phase shifting circuit 310C.The phase inverting circuit 180 has a predetermined amplification degreedepending on a ratio of the resistance between the two resistors 184 and186, so that a gain larger than 1 can be obtained by setting theresistance value of the resistor 186 to be larger than the resistancevalue of the resistor 184.

Due to the fact that the two phase shifting circuits 310C achieve 180°phase shifting at a predetermined frequency and that the phase isinverted by the phase inverting circuit 180, the phase makes a 360°phase shifting cycle as a whole, ensuring a predetermined oscillation.

[Eighth Variant of Oscillation Circuit]

Although the oscillation circuit 10E shown in FIG. 21 comprised twocascaded phase shifting circuits 310C by way of example, the oscillatingaction could also be achieved by cascading two phase shifting circuits330C as shown in FIG. 22.

An oscillation circuit 10F shown in FIG. 22 allows the two phaseshifting circuits 330C to perform 180° phase shifting at a predeterminedfrequency, as well as a phase inversion achieved by the phase invertingcircuit 180, with the result that the phase makes a 360° phase shiftingcycle as a whole, ensuring a predetermined oscillation.

By the way, the oscillation circuits 10D, 10E and 10F shown in FIGS. 20to 22 each comprised two phase shifting circuits each including the CRcircuit, although each phase shifting circuit may include an LR circuit.In the oscillation circuit 10D shown in FIG. 20 for example, theanterior phase shifting circuit 310C may be substituted by a phaseshifting circuit obtained by excluding the voltage dividing circuit fromthe phase shifting circuit 110L shown in FIG. 14, or alternatively theposterior phase shifting circuit 330C may be substituted by a phaseshifting circuit obtained by excluding the voltage dividing circuit fromthe phase shifting circuit 130L shown in FIG. 15.

Although the oscillation circuits 10E and 10F shown in FIGS. 21 and 22vary the voltage dividing ratio of the voltage dividing circuit 160 inresponse to the control voltage output from the AGC circuit 16 shown inFIG. 3, the gain of the phase inverting circuit 180 may be varied inresponse to the control voltage from the AGC circuit 16. In this case,the output from the phase shifting circuit 130C may be fed directly backto the anterior stage, with the exclusion of the voltage dividingcircuit 160 posterior to the phase shifting circuit 130C. Alternatively,the resistance value of the resistor 162 within the voltage dividingcircuit 160 may be reduced to an extremely small value with the voltagedividing ratio equal to 1.

In the case of the oscillation circuits 10D, 10E and 10F shown in FIGS.20 to 22, an input signal was fed to the input circuit 14 disposed infront of the anterior phase shifting circuit. However, the point to feedthe input signal is not limited to the front side of the anterior phaseshifting circuit, but instead the input signal may be fed to the inputcircuit 14 interposed between the two phase shifting circuits forexample.

Besides, the point to connect the non-inverting circuit 150 and thephase inverting circuit 180 shown in FIGS. 20 to 22 is not limited tothe front side of the anterior phase shifting circuit, but instead theymay be provided between the two phase shifting circuits or on the rearside of the posterior phase shifting circuit.

[Ninth Variant of Oscillation Circuit]

All the first to eighth variants of the oscillation circuit describedhereinabove comprised the phase shifting circuits each including theoperational amplifier, although each phase shifting circuit may includea transistor in place of the operational amplifier.

An oscillation circuit 10G shown in FIG. 23 comprises two phase shiftingcircuits 410C and 430C each allowing a predetermined amount of phaseshifting of an input AC signal to thereby achieve in cooperation a totalof 360° phase shifting at a predetermined frequency, a non-invertingcircuit 450 for providing through the amplification of a predeterminedamplification degree an output signal from the phase shifting circuit430C without changing the phase of the signal, a voltage dividingcircuit 160 including resistors 162 and 164 disposed posterior to thenon-inverting circuit 450, and a feedback resistor 170.

The anterior phase shifting circuit 410C shown in FIG. 23 generatesin-phase and anti phase signals from an input signal by means of an FET412 and combines these two signals by way of a capacitor 414 or aresistor 416 to obtain an output signal.

The transfer function of the phase shifting circuit 410C can intactly beK1 given by the expression (5) where T₁ is a time constant of a CRcircuit consisting of the capacitor 414 and the resistor 416 (note thata₁ <1). The phase-shift amount is also equal to that of the phaseshifting circuit 110C shown in FIG. 13.

On the other hand, the posterior phase shifting circuit 430C shown inFIG. 23 generates in-phase and anti phase signals from an input signalby means of an FET 432 and combines these two signals by way of aresistor 436 or a capacitor 434 to obtain an output signal.

The transfer function of the phase shifting circuit 430C can intactly beK2 given by the expression (6) where T₂ is a time constant of a CRcircuit consisting of the capacitor 434 and the resistor 436 (note thata₂ <1). The phase-shift amount is also equal to that of the phaseshifting circuit 130C shown in FIG. 13.

The non-inverting circuit 450 shown in FIG. 23 includes an FET 452connected to a resistor 454 interposed between its drain and a positivepower source and to a resistor 456 interposed between its source and theearth, a transistor 458 having a base connected to the drain of the FET452 and a collector connected via a resistor 460 to the source of theFET 452, and a resistor 462 for applying an appropriate bias voltage tothe FET 452.

When the FET 452 receives an AC signal through its gate, it provides ananti phase signal as its output through the drain. When the transistor458 receives the anti phase signal through its base, it provides as itsoutput through the collector a signal having a further inverted phase,that is, a signal in phase with the signal input to the gate of the FET452. The resultant signal is provided as an output from thenon-inverting circuit 450.

The combined phase shifting circuits 410C and 430C achieve a total of360° phase shifting at a predetermined frequency. At that time, the gainof the non-inverting circuit 450 is regulated so as to obtain a loopgain equal to or more than 1, thereby ensuring a predeterminedoscillating action.

Although the voltage dividing circuit 160 is disposed posterior to thenon-inverting circuit 450 so that the resistance value of the variableresistor 166 connected in parallel to the resistor 164 is regulated inresponse to a control voltage output from the AGC circuit 16 to providean amplitude control, the gain of the non-inverting circuit 450 may beregulated by varying the resistance value of e.g., the resistor 460within the non-inverting circuit 450.

In the case of the oscillation circuit 10G shown in FIG. 23, an inputsignal is fed to the input circuit 14 disposed in front side of theanterior phase shifting circuit 410C, although the point to feed theinput signal is not limited to the front side of the anterior phaseshifting circuit 410C, but instead the input signal may be fed forexample to the input circuit 14 interposed between the phase shiftingcircuit 410C and the phase shifting circuit 430C.

[Tenth Variant of Oscillation Circuit]

Although the oscillation circuit 10G shown in FIG. 23 comprised thephase shifting circuit 410C and the phase shifting circuit 430C eachincluding the CR circuit, the oscillation circuit may comprise a phaseshifting circuit including an LR circuit consisting of the resistor andthe inductor therewithin instead of the CR circuit.

FIG. 24 is a circuit diagram showing a configuration of the phaseshifting circuit which includes the LR circuit and which is replaceablewith the anterior phase shifting circuit 410C in the oscillation circuit10G shown in FIG. 23. A phase shifting circuit 410L shown in FIG. 24 hasa configuration including an LR circuit consisting of the resistor 416and an inductor 417 in place of the CR circuit consisting of thecapacitor 414 and the resistor 416 within the anterior phase shiftingcircuit 410C shown in FIG. 23. The resistors 418 and 420 are so set asto have the same resistance value. A capacitor 419 is interposed betweenthe inductor 417 and the drain of the FET 412 and serves to block adirect current.

FIG. 25 is a circuit diagram showing another configuration of the phaseshifting circuit which includes the LR circuit and which is replaceablewith the posterior phase shifting circuit 430C in the oscillationcircuit 10G shown in FIG. 23. A phase shifting circuit 430L shown inFIG. 25 has a configuration including an LR circuit consisting of theresistor 436 and an inductor 437 in place of the CR circuit consistingof the capacitor 434 and the resistor 436 within the posterior phaseshifting circuit 430C shown in FIG. 23. The resistors 438 and 440 are soset as to have the same resistance value. A capacitor 439 is interposedbetween the resistor 436 and the drain of the FET 432 and serves toblock a direct current.

In this manner, the phase shifting circuit 410C and/or the phaseshifting circuit 430C shown in FIG. 23 can be replaced by the phaseshifting circuit 410L and/or the phase shifting circuit 430L shown inFIGS. 24, 25.

[Eleventh Variant of Oscillation Circuit]

Although cascaded in FIG. 23 are two phase shifting circuits 410C and430C having different phase shifting directions, the oscillating actionmay be achieved by cascading two phase shifting circuits 410C or twophase shifting circuits 430C.

FIG. 26 is a circuit diagram showing an eleventh variant in the form ofan oscillation circuit 10H comprising two cascaded phase shiftingcircuits 410C. FIG. 27 is a circuit diagram of an oscillation circuit10J comprising two cascaded phase shifting circuits 430C.

A phase inverting circuit 480 making up the oscillation circuits 10H and10J includes an FET 482 connected to a resistor 484 interposed betweenits drain and a positive power source and connected to a resistor 486interposed between its source and the earth, and a resistor 488 forapplying a predetermined bias voltage to a gate of the FET 482. Thephase inverting circuit 480 has a predetermined gain depending on aresistance ratio between the two resistors 484 and 486.

The combined two phase shifting circuits 410C or the combined two phaseshifting circuits 430C achieve a total of 180° phase shifting at apredetermined frequency, with a loop gain set to 1 or more through theregulation of the gain of the phase inverting circuit 480, to therebyensure a predetermined oscillating action.

Incidentally, the oscillation circuits 10G, 10H and 10J shown in FIGS.23, 26 and 27, respectively, comprise two phase shifting circuits and anon-inverting circuit or two phase shifting circuits and a phaseinverting circuit so that the combined three circuits achieve incooperation a total of 360° phase shifting at a predetermined frequencyto thereby ensure a predetermined oscillating action. Therefore, whenattention is given to only the phase-shift amount, the order ofconnection of the three circuits has a certain degree of freedom,allowing the order of connection to be determined as needed.

Although the above-described oscillation circuits 10H and 10J comprisedthe phase shifting circuits including the CR circuits therewithin by wayof example, the oscillation circuit may comprise cascaded phase shiftingcircuits each including an LR circuit.

Besides, in the oscillation circuits 10H and 10J shown in FIGS. 26 and27, respectively, the voltage dividing ratio of the voltage dividingcircuit 160 is varied depending on a control voltage output from the AGCcircuit 16. Instead, however, the gain of the phase inverting circuit480 may be regulated by varying the resistance value of, e.g., theresistor 460 within the phase inverting circuit 480 in response to thecontrol voltage from the AGC circuit 16.

In the case of the oscillation circuit 10H and 10J, an input signal isfed to the input circuit 14 disposed in front side of the anterior phaseshifting circuit, although the point to feed the input signal is notlimited to the front side of the anterior phase shifting circuit, butinstead the input signal may be fed for example to the input circuit 14interposed between the anterior phase shifting circuit and the posteriorphase shifting circuit.

[Twelfth Variant of Oscillation Circuit]

FIG. 28 is a circuit diagram showing a twelfth variant of theoscillation circuit. The oscillation circuit designated at 10K in thediagram comprises a non-inverting circuit 550 for providing as itsoutput an input AC signal without inverting its phase, two phaseshifting circuits 510C and 530C each allowing a predetermined amount ofphase shifting of the input signal to achieve in cooperation a total of360° phase shifting, a voltage dividing circuit 160 consisting of theresistors 162 and 164 disposed posterior to the posterior phase shiftingcircuit 530C, and a feedback resistor 170.

It is to be appreciated that the non-inverting circuit 550 functions asa buffer circuit. This non-inverting circuit 550 may be excluded fromthe oscillation circuit.

The anterior phase shifting circuit 510C shown in FIG. 28 includes adifferential amplifier 512 which amplifies for output a differentialvoltage of two inputs with a predetermined amplification degree, acapacitor 514 and a resistor 516 (which make up a second series circuit)which allow a predetermined amount of phase shifting of a signal inputto the phase shifting circuit 510C for the input to the non-invertinginput terminal of the differential amplifier 512, and resistors 518 and520 (which make up a first series circuit) which divide the voltagelevel of the input signal in about half without inverting its phase forthe input to the inverting input terminal of the differential amplifier512.

The transfer function of this phase shifting circuit 510C can intactlybe K1 given by the expression (5) where T₁ is a time constant of the CRcircuit consisting of the capacitor 514 and the resistor 516. Thephase-shift amount is also equal to that of the phase shifting circuit110C shown in FIG. 13.

On the other hand, the posterior phase shifting circuit 530C shown inFIG. 28 includes a differential amplifier 532 which amplifies for outputa differential voltage of two inputs with a predetermined amplificationdegree, a resistor 536 and a capacitor 534 (which make up a secondseries circuit) which allow a predetermined amount of phase shifting ofa signal input to the phase shifting circuit 530C for the input to thenon-inverting input terminal of the differential amplifier 532, andresistors 538 and 540 (which make up a first series circuit) whichdivide the voltage level of the input signal in about half withoutinverting its phase for the input to the inverting input terminal of thedifferential amplifier 532.

The transfer function of this phase shifting circuit 530C can intactlybe K2 given by the expression (6) where T₂ is a time constant of the CRcircuit consisting of the resistor 536 and the capacitor 534. Thephase-shift amount is also equal to that of the phase shifting circuit130C shown in FIG. 13.

The combined phase shifting circuits 510C and 530C achieve incooperation a total of 360° phase shifting at a predetermined frequency,with a loop gain set to 1 or more through a regulation of the gain ofthe phase shifting circuit 510C and/or the phase shifting circuit 530C,to thereby ensure a predetermined oscillating action.

Besides, in case of the oscillation circuit 10K shown in FIG. 28, thevoltage dividing ratio of the voltage dividing circuit 160 is varieddepending on a control voltage output from the AGC circuit 16, althoughthe amplification degree of at least one of the differential amplifiers512 and 532 and the non-inverting circuit 550 may be varied on the basisof the control voltage from the AGC circuit 16.

In the case of the oscillation circuit 10K shown in FIG. 28, an inputsignal is fed to the input circuit 14 disposed in front side of theanterior phase shifting circuit 510C, although the point to feed theinput signal is not limited to the front side of the anterior phaseshifting circuit 510C, but instead the input signal may be fed forexample to the input circuit 14 interposed between the phase shiftingcircuit 510C and the phase shifting circuit 530C.

[Thirteenth Variant of Oscillation Circuit]

Although the oscillation circuit 10K shown in FIG. 28 comprised thephase shifting circuits 510C and 530C each including the CR circuit, theoscillation circuit may comprise phase shifting circuits including LRcircuits each consisting of a resistor and an inductor in place of theCR circuits.

FIG. 29 is a circuit diagram showing another configuration of a phaseshifting circuit including the LR circuit. The phase shifting circuitdesignated at 510L in the diagram has a configuration including the LRcircuit consisting of the resistor 516 and an inductor 517 in place ofthe CR circuit consisting of the capacitor 514 and the resistor 516within the phase shifting circuit 510C shown in FIG. 28.

FIG. 30 is a circuit diagram showing a further configuration of a phaseshifting circuit including the LR circuit. The phase shifting circuitdesignated at 530L in the diagram has a configuration including the LRcircuit consisting of an inductor 537 and the resistor 536 in place ofthe CR circuit consisting of the resistor 536 and the capacitor 534within the phase shifting circuit 530C shown in FIG. 28.

In this manner, both or either of the two phase shifting circuits 510Cand 530C can be substituted by the phase shifting circuit 510L of FIG.29 and/or the phase shifting circuit 530L of FIG. 30.

[Fourteenth Variant of Oscillation Circuit]

Although cascaded in FIG. 28 are two phase shifting circuits 510C and530C having different phase shifting directions, the oscillating actionmay be achieved by cascading two phase shifting circuits 510C or twophase shifting circuits 530C.

FIG. 31 is a circuit diagram showing a fourteenth variant in the form ofan oscillation circuit 10L comprising two cascaded phase shiftingcircuits 510C. FIG. 32 is a circuit diagram of an oscillation circuit10M comprising two cascaded phase shifting circuits 530C.

The combined two phase shifting circuits 510C or the combined two phaseshifting circuits 530C achieve a total of 180° phase shifting at apredetermined frequency, with a loop gain set to 1 or more through theregulation of the gain of at least one of the phase shifting circuits510C, 530C and the phase inverting circuit 580, to thereby ensure apredetermined oscillating action.

Incidentally, the oscillation circuits 10K, 10L and 10M shown in FIGS.28, 31 and 32, respectively, comprise two phase shifting circuits and anon-inverting circuit or two phase shifting circuits and a phaseinverting circuit so that the combined three circuits achieve incooperation a total of 360° phase shifting at a predetermined frequencyto thereby ensure a predetermined oscillating action. Therefore, whenattention is given to only the phase-shift amount, the order ofconnection of the three circuits has a certain degree of freedom,allowing the order of connection to be determined as needed.

It is to be noted that, in case of oscillation circuits 10L and 10Mshown in FIGS. 31 and 32, respectively, the voltage dividing ratio ofthe voltage dividing circuit 160 is varied in response to a controlvoltage output from the AGC circuit 16. Instead, however, theamplification degree of at least one of the differential amplifiers 512,532 and a phase inverting circuit 580 may be varied in response to thecontrol voltage from the AGC circuit 16.

In the case of the oscillation circuit 10L and 10M shown in FIGS. 31,32, an input signal is fed to the input circuit 14 disposed in frontside of the anterior phase shifting circuit, although the point to feedthe input signal is not limited to the front side of the anterior phaseshifting circuit, but instead the input signal may be fed for example tothe input circuit 14 interposed between the anterior phase shiftingcircuit and the posterior phase shifting circuit.

Although the oscillation circuits 10K, 10L and 10M shown in FIGS. 28, 31and 32, respectively, comprise cascaded phase shifting circuits eachincluding the CR circuit, at least one of the phase shifting circuitsmay include an LR circuit therewithin.

[Other Variants]

In case the various oscillation circuits illustrated in FIG. 13 andsubsequent diagrams are used in the tuning amplifier 1 having the PLLconfiguration of FIG. 2, it will be sufficient for example that theresistor 116 included in the phase shifting circuit 110C, etc., issubstituted by a variable resistor and that the resistance value of thevariable resistor is varied depending on an output from the low passfilter 5 shown in FIG. 2. More specifically, the variable resistor isformed from a channel resistor of the FET so that the gate voltage ofthe FET is controlled in accordance with the output of the low passfilter 5 shown in FIG. 2. Alternatively, the resistor 136, etc., withinthe posterior phase shifting circuit 130C, etc., may be substituted by avariable resistor formed from the FET, with the resistor 116 remainingin the form of the fixed resistor.

Alternatively, the anterior and posterior phase shifting circuits mayeach include a variable resistor. This is advantageous in that a largertotal amount of variation of the oscillation frequency, namely, a largervariable range of the oscillation frequency can be achieved due to thesimultaneous variation of the phase-shift amount of the two phaseshifting circuits.

The entire oscillation frequency may be varied by altering thecapacitance of the capacitor 114, etc., with the resistance values ofthe resistors 116 and 136 remaining fixed. For example, the capacitor114, etc., included in at least one of the two phase shifting circuitsmay be replaced by a variable capacitance element so that a variation ofthis capacitance can cause a variation of the phase-shift amountachieved by each phase shifting circuit to thereby vary the oscillationfrequency. More specifically, the above variable capacitance element maybe formed from a variable capacitance diode capable of varying abackward bias voltage applied to between the anode and the cathode orfrom an FET capable of varying a gate capacitance by the gate voltage.In order to vary the backward bias voltage applied to the variablecapacitance element, a capacitor for blocking direct current has only tobe connected in series to the variable capacitance element.

It is to be appreciated that there may be inverted the order ofconnection of the two phase shifting circuits constituting the variousoscillation circuits shown in FIG. 13 and subsequent diagrams.

The various oscillation circuits shown in FIGS. 13 to 22 achieves a highstability through the use of the phase shifting circuit 110C, etc.,using the operational amplifiers. However, differential amplifiershaving a predetermined amplification degree may be used in place of theoperational amplifiers within each phase shifting circuit since so higha performance is not required for the offset voltage or the voltage gainin a way of use of the phase shifting circuit 110C, etc., of thisembodiment.

FIG. 33 is a circuit diagram of a part necessary for the action of thephase shifting circuit, extracted from the configuration of theoperational amplifier, with its entirety acting as a differentialamplifier having a predetermined amplification degree. The differentialamplifier shown in the diagram includes a differential input stage 100formed from an FET, a constant current circuit 102 for feeding aconstant current to the differential input stage 100, a bias circuit 104for applying a predetermined bias voltage to the constant currentcircuit 102, and an output amplifier 106 connected to the differentialinput stage 100. As is apparent from the diagram, a multi-stageamplifier circuit for a voltage gain included in the actual operationalamplifier is excluded to simplify the configuration of the differentialamplifier to achieve a broader band. Such a simplified circuit allows arise of the upper limit of the operating frequency, so that there can beraised accordingly the upper limit of the oscillation frequency of theoscillation circuit 10A, etc., using this differential amplifier.

It is to be understood that the present invention is not restricted tothe above embodiments and that various modifications can be madedeparting from the scope of the present invention.

For example, in the description of the PLL configuration shown in FIG.2, the oscillation circuit 10 within the tuning amplifier 1 had anoscillation frequency variable depending on a control voltage. In caseof using an oscillation circuit having an oscillation frequency variabledepending on a control current, the control voltage has only to beconverted into a control current.

Specific examples of the phase-shifting type oscillation circuit includein addition to the circuit shown in FIG. 3 a twin T type CR oscillationcircuit, a positive-feedback applied bridged-T active BPF oscillationcircuit, and a Wien bridge oscillation circuit.

INDUSTRIAL APPLICABILITY

According to the present invention as described hereinabove, upon theinput of a signal to the oscillation circuit in oscillation mode, theoscillation output is drawn into a frequency of the input signal for apredetermined oscillating action. Due to a regulation of the outputamplitude effected by a gain control circuit in particular, there occursno variation in gain even when the tuning frequency has been altered byvarying the oscillation frequency of the oscillation circuit. Aregulation of the response speed of the gain control circuit enables theoscillating action to be performed in response to various input ACsignals such as an AM modulated signal and an FM modulated signal. Thesignal amplitude can also be amplified simultaneously with theoscillating action.

By the employment of a PLL configuration including the above oscillationcircuit in the form of a voltage controlled oscillation circuit, theoscillation frequency can easily be stabilized. In particular, the aboveoscillation circuit performs a predetermined oscillating action in theabsence of input signals, so that the PLL control can be providedirrespective of the presence of the input signals.

What is claimed is:
 1. A tuning amplifier comprising:an oscillationcircuit for performing an oscillating action at a predeterminedfrequency, said oscillating circuit allowing its output to be fed backto its input side to form a closed loop; a gain control circuit forproviding a control of an output amplitude of said oscillation circuit;and an input circuit for feeding an amplitude modulated signal to a partof said closed loop of said oscillation circuit, wherein said tuningamplifier extracts frequency components in the vicinity of anoscillation frequency of said oscillation circuit from signals fed bysaid input circuit so that amplification is made of amplitude modifiedcomponents contained in signals having these frequency components.
 2. Atuning amplifier comprising:an oscillation circuit for performing anoscillating action at a predetermined frequency, said oscillatingcircuit allowing its output to be fed back to its input side to form aclosed loop; a gain control circuit for providing a control of an outputamplitude of said oscillation circuit; and an input circuit for feedingan amplitude modulated signal to a part of said closed loop of saidoscillation circuit, wherein said tuning amplifier extracts frequencycomponents in the vicinity of an oscillation frequency of saidoscillation circuit from signals fed by said input circuit so thatamplification is made of amplitude modified components contained insignals having these frequency components, wherein said oscillationcircuit is a voltage controlled oscillation circuit having anoscillation frequency defined in response to a control voltage, with aPLL configuration including said voltage controlled oscillation circuitbeing employed to stabilize a tuning frequency.
 3. A tuning amplifieraccording to claim 1, wherein said oscillation circuit comprises anamplifier circuit and a feedback circuit, at least one of which has afrequency selective characteristic and which are connected to each otherto form a loop, and whereinsaid oscillation circuit performs anoscillating action at a predetermined frequency defined depending onsaid frequency selective characteristic.
 4. A tuning amplifiercomprising:an oscillation circuit for performing an oscillating actionat a predetermined frequency, said oscillating circuit allowing itsoutput to be fed back to its input side to form a closed loop; a gaincontrol circuit for providing a control of an output amplitude of saidoscillation circuit; and an input circuit for feeding an amplitudemodulated signal to a part of said closed loop of said oscillationcircuit, wherein said tuning amplifier extracts frequency components inthe vicinity of an oscillation frequency of said oscillation circuitfrom signals fed by said input circuit so that amplification is made ofamplitude modified components contained in signals having thesefrequency components, wherein said oscillation circuit comprises anamplifier circuit and a feedback circuit, at least one of which has afrequency selective characteristic and which are connected to each otherto form a loop, and wherein said oscillation circuit performs anoscillating action at a predetermined frequency defined depending onsaid frequency selective characteristic, and wherein when a frequency ofa signal fed by said input circuit coincides with said predeterminedfrequency, said feedback circuit shifts the phase of this signal by180°, and wherein said amplifier circuit inverts and amplifies a signaloutput from said feedback circuit for the output.
 5. A tuning amplifieraccording to claim 4, whereinsaid feedback circuit comprises a pluralityof cascaded low pass filters each including a resistor and a reactanceelement in the form of a capacitor or an inductor.
 6. A tuning amplifieraccording to claim 4, whereinsaid feedback circuit comprises a pluralityof cascaded high pass filters each including a resistor and a reactanceelement in the form of a capacitor or an inductor.
 7. A tuning amplifiercomprising:an oscillation circuit for performing an oscillating actionat a predetermined frequency, said oscillating circuit allowing itsoutput to be fed back to its input side to form a closed loop; gaincontrol circuit for providing a control of an output amplitude of saidoscillation circuit; and an input circuit for feeding an amplitudemodulated signal to a part of said closed loop of said oscillationcircuit, wherein said tuning amplifier extracts frequency components inthe vicinity of an oscillation frequency of said oscillation circuitfrom signals fed by said input circuit so that amplification is made ofamplitude modified components contained in signals having thesefrequency components, wherein said oscillation circuit comprises anamplifier circuit and a feedback circuit, at least one of which has afrequency selective characteristic and which are connected to each otherto form a loop, and wherein said oscillation circuit performs anoscillating action at a predetermined frequency defined depending onsaid frequency selective characteristic, and wherein said amplifiercircuit includes a CMOS inverter circuit.
 8. A tuning amplifieraccording to claim 7, wherein said amplifier circuit has a firstresistor connected in series to an input terminal of said invertercircuit and a second resistor interposed between said input terminal andan output terminal of said inverter circuit, with a resistance ratiobetween said first and second resistors being variable in response to anoutput from said gain control circuit.
 9. A tuning amplifiercomprising:an oscillation circuit for performing an oscillating actionat a predetermined frequency, said oscillating circuit allowing itsoutput to be fed back to its input side to form a closed loop; a gaincontrol circuit for providing a control of an output amplitude of saidoscillation circuit; and an input circuit for feeding an amplitudemodulated signal to a part of said closed loop of said oscillationcircuit, wherein said tuning amplifier extracts frequency components inthe vicinity of an oscillation frequency of said oscillation circuitfrom signals fed by said input circuit so that amplification is made ofamplitude modified components contained in signals having thesefrequency components, wherein said oscillation circuit comprises twophase shifting circuits connected to each other to form a loop andincluding a differential amplifier, of which output is fed back to theinput side, with an output of either of said two phase shifting circuitsbeing provided as an oscillation signal for output.
 10. A tuningamplifier according to claim 9, whereinat least one of said two cascadedphase shifting circuits includes a differential amplifier having aninverting input terminal to which is connected one end of a firstresistor and receiving an AC signal by way of said first resistor, asecond resistor interposed between said inverting input terminal of saiddifferential amplifier and an output terminal thereof, and a seriescircuit connected to the other end of said first resistor and consistingof a third resistor and a reactance element in the form of a capacitoror an inductor, with a connection between said third resistor and saidreactance element being connected to a non-inverting input terminal ofsaid differential amplifier.
 11. A tuning amplifier according to claim9, whereinat least one of said two cascaded phase shifting circuitsincludes a differential amplifier having an inverting input terminal towhich is connected one end of a first resistor and receiving an ACsignal by way of said first resistor, a first voltage dividing circuitconnected to an output terminal of said differential amplifier, a secondresistor interposed between an output end of said first voltage dividingcircuit and said inverting input terminal of said differentialamplifier, and a series circuit connected to the other end of said firstresistor and consisting of a third resistor and a reactance element inthe form of a capacitor or an inductor, with a connection between saidthird resistor and said reactance element being connected to anon-inverting input terminal of said differential amplifier.
 12. Atuning amplifier according to claim 9, whereinat least one of said twocascaded phase shifting circuits includes a differential amplifierhaving an inversion input terminal to which is connected one end of afirst resistor and receiving an AC signal by way of said first resistor,a second resistor interposed between said inversion input terminal ofsaid differential amplifier and an output terminal thereof, a thirdresistor having one end connected to said inversion input terminal ofsaid differential amplifier and the other end connected to the ground,and a series circuit connected to the other end of said first resistorand consisting of a fourth resistor and a reactance element in the formof a capacitor or an inductor, with a connection between said fourthresistor and said reactance element being connected to a non-inversioninput terminal of said differential amplifier.
 13. A tuning amplifieraccording to claim 9, whereinsaid oscillation circuit comprises anon-inverting circuit for providing as its output an input AC signalwithout inverting its phase, said non-inverting circuit being insertedinto a part of a closed loop formed by said two cascaded phase shiftingcircuits, and wherein said oscillation circuit performs an oscillatingaction at a frequency in the vicinity of a frequency allowing a total of360° phase shifting to be achieved by the combination of said twocascaded phase shifting circuits.
 14. A tuning amplifier according toclaim 9, whereinsaid oscillation circuit comprises a phase invertingcircuit for inverting an input AC signal for output, said phaseinverting circuit being inserted into a part of a closed loop formed bysaid two cascaded phase shifting circuits, and wherein said oscillationcircuit performs an oscillating action at a frequency in the vicinity ofa frequency allowing a total of 180° phase shifting to be achieved bythe combination of said two cascaded phase shifting circuits.
 15. Atuning amplifier according to claim 9, further comprising:a transistorbased follower circuit interposed between said input circuit and saidphase shifting circuit posterior to said input circuit.
 16. A tuningamplifier according to claim 9, further comprising:a second voltagedividing circuit inserted into a part of a closed loop formed by saidtwo cascaded phase shifting circuits, wherein said oscillation circuitissues as its oscillation signal an AC signal fed to said second voltagedividing circuit.
 17. A tuning amplifier comprising:an oscillationcircuit for performing an oscillating action at a predeterminedfrequency, said oscillating circuit allowing its output to be fed backto its input side to form a closed loop; a gain control circuit forproviding a control of an output amplitude of said oscillation circuit;and an input circuit for feeding an amplitude modulated signal to a partof said closed loop of said oscillation circuit, wherein said tuningamplifier extracts frequency components in the vicinity of anoscillation frequency of said oscillation circuit from signals fed bysaid input circuit so that amplification is made of amplitude modifiedcomponents contained in signals having these frequency components, andwherein said oscillation circuit comprises two phase shifting circuitseach including a series circuit consisting of a resistor and a reactanceelement in the form of a capacitor or an inductor, and a non-invertingcircuit for amplifying an input AC signal without inverting its phasefor output, and wherein said two phase shifting circuits and saidnon-inverting circuit are connected in a looped form, and wherein atleast one of said two phase shifting circuits includes conversion meansfor converting input AC signals into in-phase and anti phase AC signalsfor output, and combining means for combining one AC signal by way ofone end of said series circuit and the other AC signal by way of theother end of said series circuit.
 18. A tuning amplifier according toclaim 17, whereinsaid oscillation circuit performs an oscillating actionat a frequency in the vicinity of a frequency allowing a total of 360°phase shifting to be achieved by the combination of said two cascadedphase shifting circuits.
 19. A tuning amplifier according to claim 17,further comprising:a voltage dividing circuit inserted into a part of aclosed loop formed by said two phase shifting circuits and saidnon-inverting circuit which are cascaded, wherein said oscillationcircuit issues as its oscillation signal an AC signal fed to saidvoltage dividing circuit.
 20. A tuning amplifier according to claim 19,whereina voltage dividing ratio of said voltage dividing circuit isregulated depending on an output from said gain control circuit so thatan output amplitude of said oscillation circuit is kept at substantiallya constant level.
 21. A tuning amplifier according to claim 17, whereinagain of said non-inverting circuit is regulated depending on an outputfrom said gain control circuit so that an output amplitude of saidoscillation circuit is kept at substantially a constant level.
 22. Atuning amplifier comprising:an oscillation circuit for performing anoscillating action at a predetermined frequency, said oscillatingcircuit allowing its output to be fed back to its input side to form aclosed loop; a gain control circuit for providing a control of an outputamplitude of said oscillation circuit; and an input circuit for feedingan amplitude modulated signal to a part of said closed loop of saidoscillation circuit, wherein said tuning amplifier extracts frequencycomponents in the vicinity of an oscillation frequency of saidoscillation circuit from signals fed by said input circuit so thatamplification is made of amplitude modified components contained insignals having these frequency components, and wherein said oscillationcircuit comprises two phase shifting circuits each including a seriescircuit consisting of a resistor and a reactance element in the form ofa capacitor or an inductance, and a phase inverting circuit forinverting and amplifying an input AC signal for output, and wherein saidtwo phase shifting circuits and said phase inverting circuit areconnected in a looped form, and wherein at least one of said two phaseshifting circuits includes conversion means for converting input ACsignals into in-phase and anti phase AC signals for output, andcombining means for combining one AC signal by way of one end of saidseries circuit and the other AC signal by way of the other end of saidseries circuit.
 23. A tuning amplifier according to claim 22,whereinsaid oscillation circuit performs an oscillating action at afrequency in the vicinity of a frequency allowing a total of 180° phaseshifting to be achieved by the combination of said two cascaded phaseshifting circuits.
 24. A tuning amplifier according to claim 22, furthercomprising:a voltage dividing circuit inserted into a part of a closedloop formed by said two phase shifting circuits and said non-invertingcircuit which are cascaded, wherein said oscillation circuit issues asits oscillation signal an AC signal fed to said voltage dividingcircuit.
 25. A tuning amplifier according to claim 24, whereinthevoltage dividing ratio of said voltage dividing circuit is regulateddepending on an output from said gain control circuit so that the outputamplitude of said oscillation circuit is kept at substantially aconstant level.
 26. A tuning amplifier according to claim 22, whereinthegain of said phase inverting circuit is regulated depending on an outputfrom said gain control circuit so that the output amplitude of saidoscillation circuit is kept at substantially a constant level.
 27. Atuning amplifier according to claim 9, wherein at least one of said twophase shifting circuits included in said oscillation circuit includes afirst series circuit consisting of first and second resistors havingsubstantially the same resistance value, a second series circuitconsisting of a third resistance and a reactance element in the form ofa capacitor or an inductor, and a differential amplifier for amplifyingfor output with a predetermined amplification degree a differencebetween a potential at a connection of said first and second resistorsconstituting said first series circuit and a potential at a connectionof said third resistor and said reactance element constituting saidsecond series circuit, with an AC signal being fed to one end of saidfirst and second series circuits.
 28. A tuning amplifier according toclaim 27, whereinsaid oscillation circuit comprises a non-invertingcircuit for providing as its output an input AC signal without invertingits phase, said non-inverting circuit being inserted into a part of aclosed loop formed by said two cascaded phase shifting circuits, andwherein said oscillation circuit performs an oscillating action at afrequency in the vicinity of a frequency allowing a total of 360° phaseshifting to be achieved by the combination of said two cascaded phaseshifting circuits.
 29. A tuning amplifier according to claim 27,whereinsaid oscillation circuit comprises a phase inverting circuit forinverting an input AC signal for output, said phase inverting circuitbeing inserted into a part of a closed loop formed by said two cascadedphase shifting circuits, and wherein said oscillation circuit performsan oscillating action at a frequency in the vicinity of a frequencyallowing a total of 180° phase shifting to be achieved by thecombination of said two cascaded phase shifting circuits.
 30. A tuningamplifier according to claim 27, further comprising:a voltage dividingcircuit inserted into a part of a closed loop formed by said twocascaded phase shifting circuits, wherein said oscillation circuitissues as its oscillation signal an AC signal fed to said voltagedividing circuit.
 31. A tuning amplifier according to claim 30,whereinthe voltage dividing ratio of said voltage dividing circuit isregulated depending on an output from said gain control circuit so thatthe output amplitude of said oscillation circuit is kept atsubstantially a constant level.
 32. A tuning amplifier according toclaim 27, whereinthe gain of said differential amplifier is regulateddepending on an output from said gain control circuit so that the outputamplitude of said oscillation circuit is kept at substantially aconstant level.
 33. A tuning amplifier according to claim 28, whereinthegain of said non-inverting circuit is regulated depending on an outputfrom said gain control circuit so that the output amplitude of saidoscillation circuit is kept at substantially a constant level.
 34. Atuning amplifier according to claim 29, whereinthe gain of said phaseinverting circuit is regulated depending on an output from said gaincontrol circuit so that the output amplitude of said oscillation circuitis kept at substantially a constant level.